1. Field of the Invention
The present invention relates generally to a lens barrel including a plurality of lens groups such as a zoom lens or the like and, more particularly, to a lens barrel having an improved device for moving the lens groups.
2. Related Background Art
A wide-spread method of moving each lens group to a predetermined position in this type of lens barrel has hitherto involved the use of guide slots and pins. According to such a method, a lens holding ring for holding the lenses is tilted due to a fitting backlash caused when its outer diametrical portion is fitted to the other fitting part. It is therefore required that a sufficient fitting length be taken so as not to cause a decline of optical performance. Further, when the plurality of lens holding rings are fitted to the inner diametrical portion of the same member, the lens barrel is constructed in such a way that the fitting part of each lens holding ring is split in the peripheral direction, and these split segments are mutually set in gaps of the other fitting member to ensure the fitting length.
FIG. 3 is a sectional view showing a conventional example of the lens barrel. FIG. 4 is a sectional view illustrating how the lens holding ring is split in the conventional example. FIG. 5 is a development elevation showing the lens holding ring in the conventional example.
A fixed drum 51 includes a mount unit, provided at its rear part, for connecting a camera body (unillustrated). A cam ring 52 is fitted to the inner portion of the fixed drum 51. A cam pin 57 embedded in the fixed drum 51 is fitted in a cam slot 52a formed in the cam ring 52, whereby the cam ring 52 is movable in the optical-axis directions while rotating about the optical axis.
A lens holding ring 53 is a member for holding a lens group I and is fitted in the inner portion of the cam ring 52. A cam pin 55 is embedded in this lens holding ring 53. The cam pin 55 penetrating a cam slot 52b engages with a rectilinear slot 51a of the fixed drum 51. The lens holding ring 53 is movable in the optical-axis directions with the aid of the cam pin 55, the cam slot 52b and the rectilinear slot 51a.
A lens holding ring 54 is a member for holding a lens group II and is fitted to the inner portion of the cam ring 52. A cam pin 56 is embedded in this lens holding ring 54. The cam pin 56 penetrating a cam slot 52c formed in the cam ring 52 engages with a rectilinear slot 51d of the fixed drum 51. The lens holding ring 54 is movable in the optical-axis directions with the aid of the cam pin 56, the cam slot 52c and the rectilinear slot 51d.
An operation ring 58 is fitted to the outer peripheral portion of the fixed drum 51. The operation ring 58 is regulated in terms of its movements in the optical-axis directions and is so supported as to be rotatable about the optical axis. The operation ring 58 includes a projection extending in the central direction. This projection 58a penetrates run-off slots 51b, 51c of the fixed drum 51 and enters a rectilinear slot 52d.
Referring next to FIGS. 4 to 6B, the problems inherent in the prior art lens barrel will be explained in detail.
In the conventional lens barrel, the lens holding ring 54 is fitted to the inner portion of the cam ring 52. The reason for this is that the fitting length can be set larger than in the arrangement of fitting the lens holding ring 54 to the inner portion of the lens holding ring 53. In this case, the construction for preventing an interference between the lens holding ring 53 and the lens holding ring 54 is that, as illustrated in FIG. 4, the respective fitting portions are split, and the gaps are mutually filled with the split segments. This arrangement induces a greater increment in quantity of the fitting backlash than in a full-periphery fitting portion.
The reason for this will be elucidated referring to FIGS. 6A and 6B. Generally, some fitting backlash is produced with respect to the cam ring 52 so that the lens holding ring 53 can be smoothly moved. If the backlash becomes large, however, the lens holding ring 53 is tilted or becomes eccentric from the optical axis. This is conducive to worsened optical performance. Now, let d be the fitting backlash, and, when splitting the fitting part, the actual fitting backlash d' (&gt;d) is larger than the fitting backlash d.
Especially, in an optical system where the eccentric accuracy between the mutual lens groups is demanded, the conventional methods are hard to adopt. It is because the lens holding ring 53 is tilted by a quantity corresponding to the backlash d' with respect to the cam drum 52. The lens holding ring 54 is similarly tilted with respect to the cam drum 52. If tilted in opposite directions, the tilts thereof become an added value of the respective tilts enough to deviate from the demanded accuracy. The optical performance declines. Besides, the difference in attitude relative to the optical performance gets more conspicuous with a larger fitting backlash. If the difference in attitude is large, the optical performance largely fluctuates depending on the direction of the lens barrel. A quality of the product can not be ensured.
Turning to FIG. 3, a coil spring 59 is interposed between the lens holding rings 53, 54 to obviate the difference in attitude. A uniform biasing force is imparted to the lens holding rings 53, 54, with the result that they are always tilted in the same directions. The attitude-difference is thereby obviated. However, the increment in the mutual eccentric quantities still remains as it is. Further, the addition of the single coil spring 59 inevitably leads to an increase in costs.
As fully described above, according to the prior art, the fitting backlash becomes larger in the split fitting part of the lens holding ring than in the full-periphery fitting part. The eccentricity and the tilt of the lens group increase, resulting in the decline of the optical performance. Particularly, the lens system where the eccentric accuracies of the mutual lens groups are demanded presents such problems that the optical performance is hard to ensure, and the mass-productivity is insufficient.